Chronic human immunodeficiency virus type one (HIV-1) infection leads to a spectrum of neurological and cognitive abnormalities, known collectively as HIV-associated neurocognitive disorders (HAND). HAND remains prevalent, particularly in its milder forms, despite effective combination antiretroviral therapy (cART). The pathogenesis of HAND is involves HIV-infected perivascular macrophages and microglia, whose activation leads to the release of pro-inflammatory cytokines and other soluble factors toxic to neurons. One factor that may be involved in macrophage-mediated HIV neurotoxicity is cathepsin B, a cysteine protease. We recently demonstrated that monocyte-derived macrophages (MDM) secreted, cathepsin B has increased neurotoxic activity in vitro. In studies of our Hispanic women cohort, we observed increased expression of both cathepsin B and cystatin B in monocytes of women with HAD on cART with no comorbid conditions. Studies are needed to determine the role of monocyte cathepsin B, cystatins B and C, as potential biomarkers for HAND by flow cytometry. A pilot study of limited samples post-mortem brain tissue indicated that cathepsin B is also upregulated in the CNS of patients with HAND [1]. New preliminary studies of MDM secreted interactome demonstrate that cathepsin B interacts with MMP-9 in uninfected cells but this interaction disappears in HIV infection and develops a new interaction with serum amyloid P component (SAPC), related to amyloid deposition in Alzheimer's disease (AD). Studies in the literature also suggest that cathepsin B might be involved in amyloid-beta (A)- related inflammatory response, which results in neuronal death. This new role of cathepsin B in HIV infection deserves further studies. Our long-term goal is to determine the potential of the cathepsin B system as a diagnostic tool and therapeutic target for HAND and the mechanisms whereby cathepsin B dysregulation causes neuronal cell dysfunction and death. Our central hypothesis is that increased secretion of monocyte-derived cathepsin B after HIV infection causes neuronal dysfunction and death, and contributes to the pathogenesis of HAND. The proposed research is conceptually innovative because the role of the cathepsin B as a potential early blood biomarker of HAND has not been explored. Understanding how dysregulation of the cathepsin B interacting proteins in the HIV-infected MDM secretome contributes to neurotoxicity using primary human neurons could elucidate novel targets for therapy. The specific aims are: 1) Create a longitudinal expression profile of cathepsin B and cystatins B and C in CD14+ monocytes from HAND patients. 2) Characterize of the interactome of MDM-secreted cathepsin B and its involvement in HIV-induced neuronal death; 3) Determine the translational significance of cathepsin B interactome in human post-mortem brain tissue. The proposed studies will significantly advance the HIV field by providing new clinical diagnostic tools, new pathways, and possible complementary therapies against HAND.